An oscillator produces a periodic time-varying output signal. The output signal may be used, for example, as an information-carrying signal or as a timing signal for signal processing circuits. A variable-frequency oscillator (VFO) is an oscillator that produces an output signal at a frequency that can be varied controllably over a range of frequencies. A voltage-controlled oscillator (VCO) is a variable-frequency oscillator that produces an output signal with a variable frequency that is controlled by an applied tuning voltage.
A common type of variable-frequency oscillator is an LC voltage-controlled oscillator, which includes a tank circuit coupled in a feedback loop with an amplifier. The tank circuit includes an inductor and a variable capacitance element, which typically is implemented by a varactor diode. Both the inductor and the variable capacitor element, however, are associated with low quality values (Q-values), which tend to degrade the performance of the oscillator by increasing the phase noise, especially at high frequencies. The Q-value measures the ratio between the stored energy and lost energy per oscillation cycle.
In an effort to eliminate the performance-degrading impact of the variable capacitor elements, varactor-free voltage-controlled oscillators have been proposed. In one such approach, a cross-coupled LC voltage-controlled oscillator includes two LC-tuned resonator circuits with different resonant frequencies and a Gilbert multiplier cell that varies the loop transmission that is applied to each resonator circuit (see, e.g., Nhat M. Nguyen et al., “A 1.8-GHz Monolithic LC Voltage-Controlled Oscillator,” IEEE Journal of Solid-State Circuits, vol. 27, no. 3, pp. 444-450, March 1992). In this approach, the oscillation frequency is varied continuously between the resonant frequencies of the two resonator circuits by varying the relative proportions of current flowing through the two resonator circuits. This approach, however, does not eliminate the performance-degrading impact of the required planar inductors, which tend to have low Q-values in monolithic circuit designs because of resistive losses and substrate losses.
What is needed is a variable-frequency oscillator that avoids the performance limitations associated with variable capacitance elements and planar inductors in the resonator portions of the oscillator.